maximizing volume

the problem is, A rain gutter is to be made from a metal sheet of width 36cm by bending up 1/4th of the sheet on both sides through angle theta. how should theta be chosen so that the gutter can carry the maximum amount of water?

the problem is, A rain gutter is to be made from a metal sheet of width 36cm by bending up 1/4th of the sheet on both sides through angle theta. how should theta be chosen so that the gutter can carry the maximum amount of water?